The invention relates to a process for the treatment of digested sewage sludge, as well as the use of the products obtained as fuel briquettes or as a supply of fertilizer components.
The elimination of sewage sludge causes considerable difficulties, particularly in connection with municipal drainage. The hitherto adopted practice of dumping the sludge into the sea is not only uneconomical due to the high transportation costs, but to protect the environment is also only possible to a limited extent.
The treatment of sewage sludge for obtaining usable end products or harmless constituents is an important problem, particularly in large cities. It is known that sewage sludge can be digested for obtaining methane, or can be stored. This involves considerable costs for the storage tanks and the removal of the digested sewage sludge still constitutes a problem. The use of concentrated sewage sludge as a soil conditioner is problematical, due to the low concentration of substances having a fertilizing action.
It is also known to concentrate the sewage sludge or the digested sewage sludge with a solids content of approximately 2 to 5% in a settling tank, until a solids content of approximately 10% by weight is obtained. The predrained or concentrated sludge can then be further concentrated to a solids content of approximately 30 to 50% by weight by means of centrifuges or filter presses. Both sludge concentration in settling tanks and draining by centrifuges and filter presses are extremely uneconomical, particularly as a considerable amount of energy is required for the final sludge drying to a residual water content below 50%, while still not leading to the elimination of the dried residual sludge.
The problem of the present invention is to treat sewage sludge and particularly digested sewage sludge in such a way that no by-products prejudicial to the environment are obtained and so that the resulting products can be advantageously used.